如何把文本(txt)内的每2行合并成一行?

如图片

 int main()
{
    char Read[20] = {0};
    char Write[100] = {0};
    int i = 0;

    fstream fr("2.txt");
    fstream fw("3.txt");

    while(fr.getline(Read,20))
    {       
        if (i % 2 == 0)
        {
            strcpy(Write,Read);
        }
        else
        {
            strcat(Write,Read);
            fw.write(Write,sizeof(Write));
            memset(Write,0,sizeof(Write));
        }
        i ++;
    }

    fr.close();
    fw.close();

    return 0;
}

用正则表达是替换
([0-9]+)\n([0-9]+) 替换成$1$2